Isolating valves of various types are known which provide a gas-tight seal, for example, by means of cantilever sealing springs fitted to steel plates. The main purpose of such valves is to prevent cooling of the boilers and chimneys during the periods when the burners cut out periodically during operation because a pre-set temperature or steam pressure has been reached. Technical development in this field has led to a considerable growth in the use of these types of boilers with automatic operation, many of which are operated on an "on-off" cycle, i.e., they are always fully on or fully off. Other boilers are flame-modulated and switch over to on-off firing only during low-load periods which, with central heating and hot water boilers, are frequent during the greater part of the year.
During the off part of the cycle, cold air is drawn through the boiler and cools metal and water and flues and chimneys. The heat loss during the first fraction of the off-cycle is far greater than during the entire remainder of the off-cycle because then the temperature difference between the cold air and the still-hot metal surface is at its maximum value. This cooling effect not only wastes a substantial amount of fuel, but also causes smuts and air pollution and leads to corrosion of boiler, flues and chimneys.
A common firing characteristic of these types of boilers is that their on-off firing cycle is very short, even with oil-fired boilers. This is particularly so with gas-firing where cycles of 2 to 3 minutes and less are frequent.
Automatic isolating dampers for these comparatively small boilers must be simple in construction to be economical. They usually consist of a pivoted steel plate moved by a geared motor, and require up to half a minute and even more to move between their terminal positions. It is clear that this time is far too long because during the time taken by the damper to fully close the greatest part of the heat which might have been retained will have been lost. This loss may be further increased by further delay when starting up caused by the necessarily slow movement of the damper. The weight alone of the damper blade apparently precludes any appreciable speeding up of the movement of the damper.
Furthermore, when closed, these dampers do not provide any safety against furnace explosion or pressure waves. An aperture in the damper plate is frequently provided to guard against explosions, but this would prevent any heat preservation. Alternatively, an additional explosion door would be necessary.
Moreover, this preservation of heat is only possible if the damper or valve gives a virtually gas-tight seal when in the closed position. Even a very small leakage equivalent to 0.05% of the duct area would result in heat losses obviating any advantages gained. To eliminate or at least reduce leakage it is desirable to employ a thinner damper blade, but the thinner the damper blade the more the blade is liable to warpage and misalignment in service.
A light flexible flat disc is not suitable as a closing member since it would have to be deflected to make a seal. This would decrease the diameter of the disc or annulus, put the outer margin under compressional stress, and cause it to buckle.